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I have a GPS through-hole patch antenna, the through hole need to be connected to the bottom layer and then from there the feed-line flows to the GPS receiver. But I want the GPS receiver to be on top to be facing the sky so that means adding vias on the feed line which will change impedance, right?

I also have a question about the antenna ground plane since I'm a beginner in that field. My PCB is 4 layer with gnd plane as 2nd layer, can I use that as ground plane for the antenna or does the gnd plane of antenna has to be on the top layer directly underneath the antenna?

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A via has inductance , capacitance with inner layer pads and resistance but with Saturn's PCB Design tool, you can make it a 50 Ohm via, if that is what is needed.

Start with the effective dielectric constant for your f and material.

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If you add a transmission line to bottom also, don't forget you need to add also ground at the 3rd layer or use the 2nd layer ground but calculate the new impedance. It is also recommended to add ground via's surrounding the RF via. This helps bound all grounds of the multi-layer boards together in the most critical point (the RF transition). Look at it as a coaxial cable, a single wire surrounded by ground. Most Antennas require a free area of ground and all other lines and components above and below it, look carefully on the manufacturer recommendations and work accordingly.

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